Parallel fiber plasticity.

Cerebellar long-term depression (LTD) is classically observed when climbing fibers, originating from the inferior olive, and parallel fibers, axons of granule cells, are activated repetitively and synchronously. On the basis that the climbing fiber signals errors in motor performance, LTD provides a mechanism of learning whereby inappropriate motor signals, relayed to the cerebellar cortex by parallel fibers, are selectively weakened through their repeated, close temporal association with climbing fiber activity. LTD therefore provides a cellular substrate for error-driven motor learning in the cerebellar cortex. In recent years, it has become apparent that depression at this synapse can also occur without the need for concurrent climbing fiber activation provided the parallel fibers are activated in such a way as to mobilize calcium within the Purkinje cell. A form of long-term potentiation (LTP) has also been uncovered at this synapse, which similarly relies only upon parallel fiber activation. In brain slice preparations and contrary to expectation, each of these forms of parallel fiber induced plasticity, as well as classical LTD, does not remain confined to activated parallel fibers as previously thought, but both depression and potentiation have the capacity to spread to neighboring parallel fiber synapses several tens of microns away from the activated fibers. Here, the cellular mechanisms responsible for the induction and heterosynaptic spread of parallel fiber LTP and LTD are compared to those involved in classical LTD and the physiological implications that the heterosynaptic spread of plasticity may have on cerebellar signal processing are discussed.